The present invention relates to lithography, more particularly, to a pattern-forming resist or resist material suitable for forming a high quality resist pattern on a substrate or base material, usually a semiconductor, in the production of semiconductor integrated circuits and other semiconductor devices, for example, large-scale integrated circuits and bubble memory devices. The pattern-forming material is sensitive to high-energy radiation such as electron beams, X-rays, soft X-rays, and ion beams and is thermally stable.
A plurality of pattern-forming resist materials are available for the purpose of recording the high-energy radiation described above. In addition to single resist layers, further, there are multi-layered resist coatings such as duplitized or two-layered resist coatings or three-layered resist coatings. A multi-layered resist coating is useful in the formation of fine patterns on an uneven substrate using, for example, submicron electron beam lithography. This is because, as described hereinafter, the multi-layered resist coating effectively decreases scattering of the electron beam and its adverse influence on patterning, and/or a proximity effect. The differences in effect between a single resist layer and a duplitized resist coating in the electron beam lithographic process will be described with reference to FIGS. 1 and 2.
FIG. 1 illustrates steps of a pattern formation process using a single resist layer. A substrate 1 has a single layer 2 of the resist material applied thereon. As shown in FIG. 1(b), the resist layer 2 is irradiated with a pattern of electron beams (e.sup.-). During the patterning process, the electron beams are undesirably scattered within the resist layer 2. (Scattering of electron beams is shown by arrows a and b.) Scattering a is caused due to the properties of the resist layer 2, while scattering b is caused by back scattering of the electron beams from the substrate 1.
The illustrated scattering of the electron beams adversely affects the accuracy of the resulting resist pattern. For example, if the resist layer 2 has a relatively high thickness, the resulting resist pattern will show extension of the pattern ends. Further, if the layer 2 is relatively thin, the width of the resist pattern will be increased due to back scattering of an electron beam once striking the substrate 1. In both of these cases, it is impossible or difficult to form fine resist patterns on the underlying substrate. The insufficient and undersirable patterning of the resist layer is apparent from FIG. 1(c), a cross-sectional view of the developed resist layer 12.
FIGS. 2(a) to (c) show a typical example of the use of the duplitized resist coating. The principle of the illustrated method is also applicable to a pattern-forming process of this invention. As shown in FIG. 2(a), the substrate 1 has applied thereon a lower resist layer 3 and an upper resist layer 4 thinner than the layer 3. The thicker layer 3 is sandwiched between the substrate 1 and the thinner layer 4 and is formed from an organic resin having no sensitivity to the energy radiation used during the patterning of the layer 4. The layer 3 is further effective to level an uneven surface of the underlying substrate, for example, the surface of LSI chips, and therefore is generally referred to as a leveling layer.
Upon electron beam exposure, the exposed area of the upper resist layer 4 is insolubilized due to cross-linking of the resist material (see reference number 14 of FIG. 2(b)). Development is then carried out to remove the unexposed area of the resist layer 4. The patterned resist layer 14 is obtained. The pattern of the layer 14 is transferred to the underlying layer 3 by dry etching the layer 3 through the patterned layer 14, which acts as a mask or masking element. The patterned resist layer is shown in FIG. 2(c).
As is apparent from the above description and the accompanying drawings, the duplitized resist coating (3 plus 4) is free from scattering of the electron beams, since the upper layer 4 is very thin and the lower layer 3 is not affected by the electron beams during patterning of the upper layer 4. This effectively diminishes lateral extension of the pattern width. The effect is greater along with lesser layer thicknesses of the layer 4. Further, increase of the layer thickness of the lower layer 3 diminishes the influence of back scattering of electron beams onto the patterning, thereby resulting in a decreased proximity effect. As a result, fine resist patterns with a high accuracy and a high aspect ratio can be obtained. The term "aspect ratio" used herein, as is generally recognized in the art, means the ratio of the layer thickness to the pattern width of the resist pattern. A high aspect ratio means that the resist pattern has a high accuracy of size.
However, no satisfactory resist material for the formation of the upper layer of the duplitized resist coating has yet been proposed. A prior art upper-layer forming resist material is chloromethylated polydiphenylsiloxane of the structural formula: ##STR1## which is referred to as SNR in this field. The resist material has a high resistance to oxygen plasma etching used in the etching of the underlying resist layer and shows an electron beam sensitivity of about 5 .mu.C/cm.sup.2 and a submicron resolution capability. Reference should be made to EP No. 122,398-A and M. Morita et al: "Silicone-type negative-working resist SNR (I)", 44th Symposium Preprint, 28a-T-1, Japan Society of Applied Physics, P. 243, Sept. 1983. Another prior art upper layer-forming resist material is P(SiSt-CMS) reported in N. Suzuki et al: "Resist material for duplitized structure", 44th Symposium Preprint, 26a-U-7, Japan Society of Applied Physics, P. 258, Sept. 1983. P(SiSt-CMS), namely, the copolymer of trimethylsilylstyrene and chloromethyl styrene of the structural formula: ##STR2## is resistant to oxygen plasma and has an electron beam sensitivity of about 4 .mu.C/cm.sup.2 and a submicron resolution capability. Both of these resist materials, However, cause corrosion of the underlying aluminum or other metal circuit because they contain chlorine atoms.
Another resist material suitable for the formation of the upper layer of the duplitized resist coating is reported in M. Hatzakis, J. Paraszczak and J. Shaw, in "Proceedings of the International Conference on Microlithography (Microcircuit Engineering '81, Lausanne)", P. 386 (1981). PDMS reported therein, namely, dimethylsiloxane of the structural formula: ##STR3## is resistant to oxygen plasma and has an electron beam sensitivity of about 2 .mu.C/cm.sup.2 and a resolution capability of about 0.5 .mu.m l/s. There is no problem about the corrosion of metal wiring or the circuit because PDMS does not contain a chlorine atom in its molecule. However, PDMS is generally oily or gummy at an ordinary temperature, it is difficult to obtain a uniform and thin coating of the resist.
The research staff of Fujitsu Limited found that silicone resins having a ladder structure, particularly polysilsesquioxane, is highly sensitive to high-energy radiation such as electron beams or X-rays and is highly resistant to reactive ion etching, plasma etching, sputter etching, or other dry etching and therefore is useful as a pattern-forming resist material (Japanese Unexamined Patent Publication (Kokai) No. 56-49540). The resist material can be effectively used in the formation of the upper resist layer discussed above, but there are several difficulties when it is used in such a lithography process. First, the described resist material easily hardens when its coating is heated to evaporate the solvent therefrom. The hardened resist coating is insoluble in the developing solution and therefore cannot be used in the subsequent patterning steps. Heating of the coated resist material at a relatively low temperature is not desirable since it means a longer processing time. Second, the described resist material is thermally instable and, therefore, cannot be stored for a long period without change of its properties. Third, it is impossible to prepare a monodispersed sample of the resist material which shows a high resolution capability. This is because the dependence of the solubility of the resist material or polysilsesquioxane on its hydroxyl equivalent and molecular weight makes the fractional precipitation process necessary for such preparation difficult.
Therefore, what is now desired is a high-energy radiation-sensitive pattern-forming resist material having improved sensitivity to electron beams, X-rays, proton beams and other high-energy radiation exposure, resistance to reactive ion etching, sputter etching, plasma etching, and other dry etching, improved resolution capability based on monodispersibility of the material or polymer, and thermal stability. The material should not cause corrosion of the underlying aluminum or other metal circuit if it is used in the production of semiconductor devices. Further, the material should be capable of being uniformly and thinly coated and should be effectively usable in the formation of a single resist layer as well as an upper layer of the duplitized resist coating. Use of the resist material in the formation of the three-layered resist coating is not contemplated, since the coating comprises a substrate having coated thereon a leveling layer, a plasma etching-resistant layer, and a resist layer and therefore it necessitates lots of complicated and troublesome process steps.